Gromov-Hausdorff collapse of Calabi-Yau manifolds

I will survey results concerning the behaviour of Gromov-Hausdorff convergence inside the moduli of Calabi-Yau manifolds with Ricci flat metric. I will discuss some old results with Pelham Wilson giving the first non-trivial precise description of collapse in the case of elliptically fibred K3 surfaces. I will then survey some more recent work by others and open problems.